Sensory information from the gastrointestinal (GI) tract is perceived and modulated by neurons in the nucleus of the tractus solitarius (NTS) before being transmitted to motoneurones of the dorsal motor nucleus of the vagus (DMV); these nuclei are essential for the coordination of vago-vagal reflexes. Although vago-vagal reflex control of the gut is understood at a basic mechanistic level, there are many factors (e.g. the organisms' place in the environment, time of day, taste of food, stress, pain, cytokine production in disease, hormonal background, etc.) that can radically alter feeding behavior or GI function. The integration of digestive function in relation to the state of the GI tract is dependent on visceral afferent data carried by the vagus nerve. Although several neurotransmitters are known to influence GI functions via either the NTS or the DMV, very few studies have investigated the synaptic contacts between these nuclei. Data collected from our laboratories suggest that neuropeptides and hormones can drastically alter the processing of vagal afferent input through differential gating of neuronal transduction mechanisms in the NTS. Our electrophysiological preliminary data suggest that subthreshold manipulations of vago-vagal circuits unmask responses in otherwise silent inhibitory synapses. Our immunohistochemical data support this concept of short-term circuit plasticity since similar manipulations allow the detection of otherwise concealed membrane receptors. Our in vivo data support this concept of short-term plasticity suggesting that this type of response highlights a generalized way of activating synaptic transmission as the need arises. The overall objective of this proposal is to elucidate the basic mechanisms of this type of short-term synaptic plasticity in relation to the activation state of the vagal afferents. Based on our preliminary data, we formulate the innovative hypothesis that the modulation of inhibitory neurotransmission within the dorsal vagal complex (DVC; i.e. DMV and NTS) depends on the state of activation of NTS interneurones. We will investigate the central hypothesis using i) in vitro patch-clamp techniques combined with fluorescent tracing methods to record selectively from GI-projecting neurons of the DMV in brainstem slices; ii) in vivo microinjections of putative neurotransmitters in the DMV to evoke and monitor GI motility; and iii) histological, immunohistochemical and morphological analyses of vagal brainstem GI innervation. The studies that we propose are based on two specific aims. Specific Aim 1: inhibitory neurotransmission in the DVC needs to be "primed" to respond to neuromodulators. In particular, we propose that the activation of the cAMP-Protein Kinase (PK) pathways is a necessary prerequisite for the activation of receptors in otherwise silent synapses. We will test this Specific Aim with the following hypotheses: (1): activation of the cAMP-PK pathway allows the modulation of otherwise unresponsive inhibitory currents by opioids and/or pancreatic polypeptides (PPs). (2): activation of the cAMP-PK pathway reveals otherwise undetectable peptide receptors in GABAergic terminals of the DVC. Specific Aim 2: the capacity of endogenous neurotransmitters to modulate inhibitory currents in the DVC is dependent upon the level of ongoing vagal activity. In particular, we propose that the effects of opioids and PPs depend upon whether the vagus nerve is stimulated or unstimulated or whether the preparation is from an animal that is fed or fasted. We will test this Specific Aim with the following hypotheses: (3): the in vitro DVC response to opioids or PPs depends upon the feeding status. (4): activation of vago-vagal reflexes or changes in the feeding status of the rat unmasks peptide receptors in GABAergic neurons and terminals in the DVC. (5): the in vivo DVC response to exogenously applied opioids or PPs depends upon the level of ongoing vagal activity or the feeding status of the rat. This research will provide an innovative way of viewing the modulation of vago-vagal reflexes. Data obtained from the pursuit of these studies will lead to an improved understanding of the short-term modulation of central vagal circuits by neuromodulators; the understanding of these gating mechanisms may allow us to explain, and eventually treat, their pathophysiological consequences.